1. Description of the Prior Art
Numerous objective lens systems for cameras have been proposed. Examples employing two elements include Simpson, Jr. et al., U.S. Pat. Nos. 4,932,764 and 5,000,552, and Davis, U.S. Pat. No. 2,586,418. See also Aldis, U.S. Pat. No. 682,017, discussed below. A two element lens for use in a copying machine, as opposed to a camera, is disclosed in Japanese Patent Publication No. 3-242608. Similarly, Rothe, U.S. Pat. No. 4,645,311, discloses a two element design including an aspheric surface for use in an optical scanner.
Three element designs include Bertele, U.S. Pat. No. 2,721,501, Grey, U.S. Pat. No. 3,784,287, De Jager, U.S. Pat. No. 3,967,884, and Sato, U.S. Pat. No. 4,542,961 in which an aspheric surface is used, as well as the early Cooke/Taylor triplet and variations thereof. See Kingslake, R., A History of the Photographic Lens, Academic Press, New York, 1989, pages 103-115.
The incorporation of a focusing element or group in an objective lens system is disclosed in the following patents: Westphalen, U.S. Pat. Nos. 3,185,061 and 3,388,650, Laurent, U.S. Pat. No. 4,174,153, Yamada, U.S. Pat. No. 4,394,071, Owen, Jr. et al., U.S. Pat. No. 4,443,067, Wakabayashi, U.S. Pat. No. 4,669,848, Nishi et al., U.S. Pat. No. 4,791,441, Nakayama et al., U.S. Pat. No. 4,830,474, Takase, U.S. Pat. No. 4,913,537, and Ogawa et al., U.S. Pat. No. 5,005,038. A construction of this general type for use in a viewfinder is disclosed in Wakamiya, U.S. Pat. No. 4,832,470. A magnifying lens employing an aspheric surface for attachment to the objective of a camera is disclosed in Baker, U.S. Pat. No. 3,604,786, and the use of an auxiliary lens for short range work is disclosed in Noguchi, U.S. Pat. No. 4,505,566.
Also of interest is U.S. Pat. No. 697,959, dated Apr. 22, 1902 to Ernst Abbe. He discusses how an aspheric surface can be used to improve far off-axis performance of a landscape type lens (cemented doublet and stop).
Notwithstanding the variety of objective lens systems which have been previously proposed, there still exists a need for lens systems of relatively simple construction which can be mass produced at low cost. In particular, such systems are needed in the field of multiple purpose hand cameras which are used by individuals and families, as well as for commercial applications, such as in the preparation of advertisements and the like, where small scale photographs of good to excellent quality can be useful and economic.
The present invention addresses this need and provides objective lens systems which 1) contain a minimum number of elements, 2) can be manufactured at low cost, and 3) have optical and physical properties suitable for use in a mass produced camera.
2. Design Principles
As is known from basic optical principles, even a simple, single lens element with spherical surfaces possesses a paraxial image position conjugate to an object distance, and an equivalent focal length, which together comprise the element's "Gaussian" optics of first order magnitude. For a rotational lens system, the usual case for practical objectives intended for mass production, there are no second order aberrations.
The chromatic variations of both image location and image size, that is to say, longitudinal and lateral chromatic aberrations, can be construed as of third order in magnitude. For the monochromatic third order of approximation and, in particular, for the development of a power series which represent the x and y intercepts on the image surface of a light ray from an object point after transit through the optical system, there will be five independent mathematical coefficients of series terms which represent the conditions to be reduced essentially to zero or at least to optimized small non-zero values. These "Seidel" coefficients in physical meaning have to do with spherical aberration, coma, astigmatism, curvature of field, and distortion.
The development of the associated power series can be carried out in explicit formulations of object distance, curvature of the object surface if any, off-axis position of the object point, intercepts of a given ray in the entrance pupil, or the equivalent in image space, and the like. Thus, the x and y intercepts of a given ray on a desired image surface can be given as a power series in 5 variables, one of which is the wavelength, or in 4 variables monochromatically, or even in 3 if rotational relationships are employed. The variables may be simple, as for example the direction of the object point as seen from the center of the entrance pupil (2 variables), and the x and y intercepts of a ray from the object point on the entrance pupil, or in terms of selected functions of the simple variables. The design task if formulated in simplest terms is to bring all rays from an object point into sharp focus at an image point. The location of the image point may not be exactly where desired but may include a displacement in x, y and z according to allowable tolerances. For example, a lateral displacement in the image surface may be permitted to some degree, in the nature of distortion, one of the Seidel conditions.
In the fifth order of approximation one encounters nine monochromatic aberrations, plus chromatic variations of the five Seidel conditions, and secondary chromatic variations of the image position in longitudinal displacement, as well as secondary chromatic variations of magnification, or of the equivalent focal length if the object plane lies at infinity. Higher orders of power series approximation have been worked out by various theorists, but the complexities of solution are such as to confine the higher order formulas generally to analysis rather than to synthesis of optical structures.
The above mentioned power series contain the basic parameters of construction of the optical system, such as the radii of curvature of the multiple surfaces, the axial spacings of the surfaces, the indices of refraction of the media used, including the wavelength dependence in some mathematical form, and if any particular surface is aspheric, that is, nonspherical, the aspheric coefficients that define the shape of the surface according to the order of approximation. The object of the invention is thus to select a set of parameters which can be used to cost-effectively to produce an objective lens system which can be successfully employed for family or general purpose applications and the like.
3. Historic Lens Designs
In the past, so-called landscape lenses have combined a single meniscus lens element with spherical surfaces with a stop at an optimum location in the lens barrel, such that one can say that the meniscus is curved in a general sense about the stop. Even if the aperture-ratio is quite limited, as for example, f/16 or slower, such a landscape lens is afflicted with distortion. With the stop forward, the distortion is barrel in nature, and if the stop is rearward of the meniscus, the distortion is pin-cushion. If the stop is rearward, the overall length is accordingly reduced. In addition, compromises have to be made with curvature of field, chromatic aberrations, astigmatism and residual aberrations of spherical aberration and coma. Nevertheless, vast numbers of family photographs have been made by means of such simplified objectives, optimized as well as may be from the complex of considerations.
In accordance with the invention, it has been found that the optical performance can be improved even for a plastic material having a relatively low index and moderate dispersion characteristics by means of at least one surface of the meniscus becoming aspheric. Use of an aspheric of optimum shape has been found to reduce distortion and to reduce astigmatism to a level sufficient to flatten the mean image surface. By means of modern manufacturing techniques known in the art the use of at least one aspheric surface per plastic element is practical for mass production.
As discussed in detail below, the photographic objective of the present invention has two lens groups, one before a centrally located stop and one after. Thus, in general terms, it can be considered to resemble that of the Aldis lens (see Aldis, U.S. Pat. No. 682,017, issued Sep. 3, 1901).
Symmetrical or nearly symmetrical objectives of two meniscus elements enclosing a central stop are known in the art and have many uses, particularly for medium to wide angle performance where a "slow" lens may be used. The well-known "Hypergon", U.S. Pat. No. 706,650 by Carl Paul Goerz, which at a high aperture-ratio such as f/60 can cover very wide angle fields on a nearly flat image surface, uses this approach. The symmetry of the arrangement eliminates most of the coma even for an infinite object distance, although not entirely. The very restrictive central stop reduces the spherical aberration and longitudinal chromatic aberrations to manageable proportions and the rest of the residual coma as well. The effectiveness of the thin Hypergon menisci arises from the near optical cancellation of the convex and adjacent concave surfaces of the individual meniscus, the power, though low, coming partly from the separation of the convex and concave surfaces, that is, from the thickness of the individual meniscus. An optimization causes the tangential and radial image surfaces to lie close to a mean nearly flat image surface and to one another as well.
A difficulty with the more or less symmetrical double meniscus form is that the principal points and indeed the perspective center lie more or less within a rather long lens barrel. The overall length of the objective from front surface vertex to the axial point of the image surface is thereby greater than the equivalent focal length, resulting usually in a lack of compactness in the camera itself. Indeed, an important requirement of the modern hand camera is compactness. Another requirement is that the clear apertures of front and rear surfaces be relatively modest which means that the menisci cannot be very far from the central stop, front and rear, if an adequate field of view is to be covered.
Unsymmetrical objectives employing at least one strong meniscus component preceding the stop, which in some cases is only a single element, are also known in the art. This structure enhances several characteristics of the front element of the Hypergon. The inner concave surface opposed to and lying at a moderate axial distance from the stop provides to some extent for a reduced Petzval sum and reduction in astigmatism caused by the meniscus component as a whole, while at the same time a substantial thickness of the meniscus component preserves much of the net lens power needed for focusing the oncoming rays from a distance object. These historic designs, however, have not achieved the optical performance at low cost which the present invention achieves.